Outboard polarizing light-transmitting means



Dec. 1970 A. D. ALEXANDER m v 3,55

OUTBOARD POLARIZING LIGHT-TRANSMITTING MEANS Filed Jan. 11, 1968 720 mp returder 05A 288 mp retarder 44 m" 0.75 A retorder RELAT|VE TRANSMISSION I725 mp retorder 432 mp retordar a v Tmm 400 432' 480 575 v e9o 720 750 WAVE LENGTH (mp) FIG. I

I 2 a 2 4) 51 support member g/ f AA elemqnt retaining flange 4 I I I I I 1 I I z United States Patent 3,551,027 OUTBOARD POLARIZING LIGHT-TRANSMITTING MEANS Arthur D. Alexander III, Cohasset, Mass., assignor to Polaroid Corporation, Cambridge, Mass, 21 corporation of Delaware Filed Jan. 11, 1968, Ser. No. 697,111 Int. Cl. G02b 5/30 US. Cl. 350-155 8 Claims ABSTRACT OF THE DISCLOSURE When light-polarizing material is used inboard of stressed window members which are exposed to sunlight the resultant polariscopic effect produces undesirable color patterns and striations. These may be relieved by placing the light-polarizing material outboard of the stressed window member.

This invention relates to the utilization of polarizers in conjunction with stressed synthetic plastic materials in a composite Window environment. More particularly, this invention relates to a combination comprising a stressed synthetic plastic window member with light-polarizing materials whereby interference spectra resulting from the polariscopic effect produced when a stressed translucent material is placed between two light-polarizers are obviated.

It is a primary object of the present invention to eliminate undesirable interference spectra associated with the polariscopic effect produced when a stressed translucent material is viewed between light-polarizing members.

It is another object of the present invention to provide a composite window structure which is particularly useful in pressurized airplanes and comprises a stressed member and a light-polarizer wherein light passing therethrough is free of chromatic aberration associated with the polariscopic effect.

It is an additional object of the present invention to provide a composite window structure which comprises a polarizing material and is substantially abrasion resistant and capable of withstanding the extreme weather conditions incident to air travel.

Other objects of the invention will in part be obvious and will in part appear hereinafter.

The invention accordingly comprises the product possessing the features, properties and the relation of components which are exemplified in the following detailed disclosure and the scope of the application of which will be indicated in the claims.

For a fuller understanding of the nature and objects of the invention reference should be had to the following detailed description taken in conjunction with the accompanying drawing wherein:

FIG. 1 is a graphical representation of wave length vs. relative transmission of white light passing through various retardation zones, T and T being the maximum and minimum light transmission possible, respectively, with a given system; and

FIG. 2 is a cutaway side view of a composite window structure within the scope of the present invention.

It is very often desirable to attenuate light entering a room or a transportation vehicle by the utilization of light-polarizing material. The need for such attenuation is generally most profound in relation to transportation vehicles and particularly airplanes which are often exposed to direct skylight without benefit of natural attenuation provided by cloud formations.

Skylight, by its nature, is rather highly polarized; that is, there is preferential alignment of the vibrating light Patented Dec. 29, 1970 waves due to particle scattering, etc. which produces a more or less ordered, rather than random wave configuration. This effect may easily be experienced by rotating 9. light-polarizer against skylight and noting the differences in intensity of the light passing through the polarizer. When the absorption axis of the polarizer is parallel to the primary plane of vibration of incident light, partial extinction occurs to a greater extent than would be achieved were the transmission axis of the polarizer aligned therewith.

In order to achieve a high degree of pressurization and safety for the interior of modern aircraft, the windows thereof are made of very highly stressed synthetic plastic materialusually polymethyl methacrylate (Plexiglas, Lucite), although polycarbonates such as Lexan, the condensation product resultant from the reaction of bisphenol A and phosgene, and diethylene glycol bis allyl carbonate, sold by Pittsburgh Plate Glass Company under the trade name CR39, or any other suitable transparent material may be used. As a failsafe procedure a second synthetic plastic window of unstressed material is ordinarily inserted inboard of the highly stressed member. In the event that a polarizer is placed inboard of said stressed member, a polariscopic effect is noted; that is, the polarized light outboard of the window and the polarizer inboard of the window form an analyzer which, in effect, vividly and chromatically portrays the stress pattern of the stressed outboard window. This effect is quite undesirable aesthetically and often causes passengers on an airplane needless worry since the pattern changes with changes in pressure.

The stress pattern chromatically portrayed in the outboard window is due to birefringence occurring in the stressed material. In polymethyl methacrylate and similar type windows the maximum retardation produced by this birefringence is up to approximately 3 Wave lengths, which is in a vividly colored area.

Retardation may be regarded as the product of hirefringence and the thickness of a given material. Thus, if the two refractive indices of a given birefringent material are denoted as n and n the birefringence as (nn )=An, and the thickness of the material is denoted at t, the retardation may be visualized as Since the thickness range and the birefringence of the members utilized in window configurations anticipated by the present invention are such that the retardations which result are of the order of magnitude of approximately 3 wave lengths, some method must be devised for causing a depolarization effect to thereby produce substantially white light, which is the subject of US. application Ser. No. 697,113, filed on the same date as the instant case in the name of Albert S. Makas, now abandoned, or for removing the system from a polariscopic environment while retaining the benefits inherent with the utilization of polarized light, which is the subject of the instant application.

It may be beneficial at this time to briefly mention the theory producing the various chromatic effects achieved with stressed synthetic plastic material of the type utilized herein in a polariscopic environment. Let us assume, for example, that at one point in said material the stress produces a birefringence which causes approximately onequarter wave length of retardation relative to 575 m light, which is in about the center of the visible spectrum, thus producing a 144 m retardation. Looking at FIG. 2, the inboard polarizer is positioned to produce maximum extinction with the incoming skylight, i.e., the absorption axis of the polarizer is parallel to the primary plane of vibration of the entering light. It will be seen, as is illustrated in FIG. 1, that for midspectrum light there will be an effect on transmission of a one-quarter wave length retardation; for 400 m light there will be 144/400 or for the 750 m light there will be approximately wave length retardation. It is evident that the resultant light will be slightly blue colored. If approximately 4 wave lengths of retardation is produced, relative to 575 my light, again with reference to FIG. 1, it may be seen that the resultant light would be approximately amber; while /2 wave length retardation relative to 575 III/.4. light produces greenish yellow light. Also included are graphical representations of the effect of 1.25 and 3.0 wave lengths of retardation, relative to 575 m light, on light which may be incident to the system of the present invention. For orders up until approximately 3.5 wave lengths retardation is achieved, very vivid simple colors are progressively evident. Over 3.5 wave lengths the colors are so profuse that they cannot be distinguished individually and are viewed as a less colorful mixture or as white light. Graphically they may be visualized as curves containing many maxima and minima.

It is known that one light-polarizing member will not cause substantial extinction of polarized light when its absorption axis is parallel to the primary plane of vibration of said light if a birefringent material is placed between said polarizer and said polarized light because the birefringent material changes the linearity of the penetrating polarized light waves and causes the emission of waves which may be helical or elliptical in nature. It has additionally been found that when an axis of stress is 45 to either transmission or absorption axes of the crossed polarizers a maximum intensity of the interference spectra is reached. Where the axis of stress coincides with either transmission or absorption polarizer axes no effect is seen. The resultant pattern, therefore, evident to a viewer, appears as 4 distinct quadrants with maximum intensity at the center of each sector and a minimum intensity along the polarizer axes.

In the copending application of Albert S. Makas, referred to above and incorporated herein by reference, it is disclosed that in an environment including a stressed translucent window member with a polarizer inboard thereof, undesirable interference spectra may be obviated by placing a high order wave plate between said polarizer and said translucent member. This technique, while causing substantial depolarization and elimination of undesirable patterns and striations, is primarily useful in situations which relate to variable density windows wherein a rotatable light-polarized member is incorporated in the system and placed in a position so that crossing of the absorption axes of the fixed and rotatable polarizers will cause complete extinction of any light attempting to pass therethrough. In order to conserve weight and provide as compact a structure for utilization in airplanes and in other related environments as possible, as well as providing a highly abrasion-resistant outer layer, it has unexpectedly been discovered that the entire undesirable polariscopic effect may be obviated by laminating the polarizer material outboard of the stressed window member.

Any suitable material which will produce the desired light-polarization effect may be utilized in the present invention. It has been found, however, that polymeric lightpolarizing sheet material lends itself most readily to this function. The preferred material is a transparent sheet of polyvinyl alcohol containing substantially oriented molecules of dehydrated polyvinyl alcohol and deriving its polarizing properties essentially from said dehydrated molecules. The manufacture and utilization of such sheet material may be appreciated with reference to US. Pats. Nos. 2,173,304; 2,255,940; 2,306,108; 2,397,231; 2,445,- 555; 2,453,186 and 2,674,159, all incorporated herein by reference.

FIG. 2 denotes an embodiment of the present invention wherein a light-polarizing material is laminated outboard of a synthetic plastic stressed window material. The structure, which will be discussed in detail below, comprises a stressed window member, 4, upon which is laminated a composite light-polarizing unit which comprises a light-polarizing material, 3, sandwiched between transparent synthetic plastic protective members, 2, and upon which is deposited an abrasion-resistant coating, 1. Member 5, is an unstressed plastic material which provides a failsafe function in the event the window structure is used in a pressurized environment. The inner member is designed to act as a seal to prevent cabin depressurization if the outer member fails.

In order to gain optimum qualities with regard to shrinkage and crazing of the light-polarizing material it has been found beneficial to laminate said light-polarizing material between layers of cellulose acetate butyrate or similar materials as described in, for example, US. Pat. No. 2,674,159 and in the applications of Buzzell and Bloom, Ser. Nos. 577,578 and 577,576, respectively, both filed on Sept. 2, 1966. Lamination of the cellulose acetate butyrate layers to the light-polarizing material may be accomplished, for example, either by subcoating the cellulose acetate butyrate material with a suitable compound such as, for example, cellulose nitrate, and then laminating the subcoated material to the polyvinyl alcohol light-polarizing sheet using a well-known pressure roll technique with a typical adhesive for such material, such material, such as a 2% polyvinyl alcohol solution; or, alternatively, the lamination may be accomplished by the conversion of the surface of the cellulose acetate butyrate to cellulose, subcoating the surface with polyvinyl alcohol and pressure laminating the subcoated cellulose acetate butyrate to the light-polarizing material, as more fully discussed in the Buzzell application mentioned above.

The instant invention may be further appreciated with reference to FIG. 2, which is the preferred embodiment, wherein a stressed window member, 4, of about 0.5 inch in thickness is placed outboard of a similarly dimensioned unstressed failsafe window member, 5. The stressed window member comprises a laminate consisting of, in order from the outboard side; an abrasion-resistant coating, 1, approximately 0.5 mil to 2 mils in thickness and preferably 0.9 mil in thickness; a first transparent cellulose acetate butyrate material, 2, approximately 430 mils in thickness and preferably about 13.5 mils thick; a polarizer member 3 as described above, approximately 0.75 1.5 mils in thickness and preferably about 0.75 mil thick; a second cellulose acetate butyrate layer, 4, of the same dimensions as said first cellulose acetate butyrate layer; and said unstressed synthetic plastic window member as described above, 5. The respective constituents of the composite window structure may be held together by any suitable mounting structure including an aperture capable of retaining the above-described light transmitting means transverse thereof, such as a cylindrical member with suitable element-retaining flanges.

The abrasion-resistant coating designated in FIG. 2 as a layer 1 may, for example, be a completely polymerized melamine formaldehyde resin, a thermosetting crosslinked polymer, such as a diethylenic substituted polyalkylene glycol, e.g., polymeric coatings formed by in situ polymerization of polyalkylene glycol diesters of c p-unsaturated carboxylic acids on the plastic sheet material, etc. Laminations of mar-resistant coatings and methods by which they may be applied are disclosed, for example, in US. Pat. Nos. 2,237,567; 2,527,400; 2,554,850; 3,081,192 and 3,097,106, all incorporated herein by reference.

Any suitable adhesive with good optical properties may be used to bond the stressed window member to the composite polarizer structure using, for example, the previously mentioned pressure roll technique. Exemplary of such adhesive materials is an adhesive which consists of approximately 4% cellulose nitrate whose viscosity is approximately 600-1000 seconds dissolved in approximately methyl methacrylate with about 1% by weight of a diisopropyl percarbonate catalyst. This adhesive material is more fully described in US. application Ser. No. 697,019 filed in the name of Harold O. Buzzell on Jan. 11, 1968.

Throughout the specification the terms inboard" and outboard have been extensively used. Out-board is considered to be the side through which light may enter the apparatus of the present invention, while inboard relates to the side through which transmitted light exits from said apparatus.

Since certain changes may be made in the above product without departing from the scope of the invention herein involved it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed:

1. A light-polarizing window apparatus having stressinduced birefringent areas and including an inboard side through which transmitted light may exit from said apparatus and an outboard side through which light may enter said apparatus comprising, in combination, a mounting structure having an aperture therein and having mounted within said aperture a composite structure comprising a light-polarizing element and a light-transmitting element having stress-induced birefringent areas, said light-polarizing element being bonded only to the outboard side of said light transmitting element and having an abrasion-resistant coating thereon, whereby said stress-induced birefringent areas are not evident to an inboard viewer of light transmitted through said composite structure from the outboard side thereof.

2. The invention of claim 1 wherein said abrasionresistant coating comprises a polyalkylene glycol diester of an a,;3-unsaturated carboxylic acid.

3. The invention of claim 2 wherein said composite structure includes, as protective layers on either side of said light-polarizing element, layers comprising cellulose acetate butyrate.

5 The invention of claim 4 wherein said light-transmitting element comprises polymethyl methacrylate.

4. The invention of claim 1 wherein said light-transmitting element comprises a synthetic plastic material.

6. The invention of claim 5 wherein said .cellulose acetate butyrate is laminated to said polymethyl methacrylate by means of an adhesive comprising approximately methyl methacrylate, 4% cellulose nitrate with a viscosity of 600 to 1,000 seconds and 1% di isopropyl percarbonate initiator.

7. The invention of claim 5 wherein said light-polarizing element comprises a layer of polyvinyl alcohol comprising substantially oriented molecules of dehydrated polyvinyl alcohol.

8. The invention of claim 7 wherein said coating com prising a polyalkylene glycol diester of an c p-unsaturated carboxylic acid is approximately 0.5 to 2 mils in thickness, said protective layers of cellulose acetate butyrate are approximately 4 to 30 mils in thickness, said lightpolarizing element is approximately 0.75 to 1.5 mils in thickness, and said light-transmitting element is approximately 0.5 inch in thickness.

References Cited UNITED STATES PATENTS 2,173,304 9/1939 Land et al. 350- 2,225,940 9/1941 Rogers 350-154X 2,306,108 12/1942 Land 350-154 2,527,400 10/1950 Cooper 350-155 2,776,598 1/1957 Dreyer 350-155 2,674,159 4/1954 Binda 350-154 2,989,787 6/1961 Smith 350-67 3,081,192 3/1963 Idelso-n 177-64 3,097,106 7/1963 Blout et al 177-64 2,255,933 9/1941 Land 350-149 3,400,972 9/1968 McIntyre et al 296-97 PAUL R. GILLIAM, Primary Examiner UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No- 3,55l,027 Dated December 29, 1970 Invented-(5) Arthur D. Alexander III It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Columfi 6, claim 4, line 1 "1" should be 3. Signed and sealed this 2nd day of May 1972.

(SEAL) Attest:

JR. ROBERT GOTTSCHALK EDWARD M'FLEICHER Commissioner of Pate Attesting Officer USCOMM-OC 6037 6-1 FORM PO-105O (10-69) 

